The scotch yoke has been used for many years as a means for converting reciprocating linear movement to rotary motion and vice versa. It has found application in a variety of machines, such as motors, pumps, and compressors which utilize a piston articulated within a closed cylinder, see, e.g., U.S. Pat. Nos. 283,558, 813,736, 999,220, and 2,628,602, as well as, in compactors, pumps, punch presses, robots, sewing machines, generators, and material handlers.
The essential components of a scotch yoke are a crankpin rotated about a crankshaft center at an axial offset and a shuttle having a slot therein through which the crankpin is positioned. The motion of the shuttle is constrained to a linear path by a guide, frequently, a pair of opposing parallel guide surfaces. The crankshaft and crankpin move in rotary motion and may be either the driven elements or the driving elements. The shuttle moves in rectilinear motion and likewise may be the driven element or the driving element. Thus, the scotch yoke provides a means for converting linear to rotary motion and vice versa.
The slot within the shuttle must be at least as wide as the crankpin diameter and long enough to accommodate the crankpin dimension and its travel. A pair of competing objectives in the design of scotch yokes is to eliminate friction, as well as clearance at the crankpin/slot interface. Friction results in energy loss in the conversion from linear to rotary motion or vice versa and also in wear of the scotch yoke. Clearance at the interface results in a loss of motion translation, commonly called "backlash", when converting from rotary to linear and vice versa (i.e., there is no translation during traversal of the clearance gap), and in brinelling, spalling and vibrations when the unrestrained driving element accelerates across the clearance gap and collides into the driven element. As has been recognized for many years, the consequences of clearance and friction at the slot/crankpin interface are energy inefficiency and excessive wear and tear.
A variety of methods have been proposed to simultaneously achieve minimum crankpin to slot clearance and friction. For example, in U.S. Pat. No. 1,687,425 a spring urged lever presses against the crankpin to eliminate excess clearance. In U.S. Pat. No. 2,366,237 the shuttle includes a bearing block having a center roller bearing for the crankpin and side roller bearings to reduce friction between the block and the remainder of the shuttle. U.S. Patent No. 4,685,342 to Douglas C. Brackett, the inventor herein, entitled DEVICE FOR CONVERTING LINEAR MOTION TO ROTARY MOTION OR VICE VERSA, discloses a scotch yoke type device having a pair of opposing, offset bearing surfaces, one on either side of the crankpin slot in the shuttle. A corresponding pair of roller bearings are arranged on the crankpin coaxially and laterally displaced from one another such that each aligns with one of the pair of opposing, offset bearing surfaces of the slot within which the crankpin is positioned. In this manner, clearance at the crankpin/slot interface can be minimized to manufacturing tolerances and friction is reduced to the rolling friction of a roller bearing.
Whereas different motion translation applications have varying requirements with respect to loading stresses, materials, cost, etc., it remains an objective in the art to create new motion translator designs having alternative structural and functional characteristics while achieving energy efficiency and wear resistance. It is therefore an object of the present invention to provide an alternative motion translator mechanism to a scotch yoke which tends to simultaneously minimize clearance and friction between the crankpin and the shuttle slot but which also retains the benefits of a scotch yoke design.